Abstract:
A dielectric superstrate or radome is desirable to protect the copper patch of the microstrip
antenna in many applications, especially for the outdoor wireless communications. The
dielectric cover offers such a protection while keeping the antenna in low profile. Many
theoretical and experimental investigations on single or stacked-patch antennas covered with a
superstrate show that the resonant/mean frequency of patch antenna decreases monotonically
with superstrate thickness [1]-[4] whereas the impedance bandwidth and radiation efficiency
also degrade with increases in superstrate thickness [5]. All these studies were made for the
linearly polarised (LP) patch antennas. Scant attention was given to the effect of superstrate on
the mutual coupling between LP patch antennas [6], and radiation characteristics of circularly
polarised (CP) antennas. Further, no studies were found on the effect of superstrate on the
mutual coupling between CP patch elements.
As reported in [8]-[10], the electrically thin dielectric material with a low dielectric
constant is preferred for the superstrate layer because the low-dielectric-constant material is
good for the patch radiation. The choice of the thickness was a balance between the undesired
effects on the impedance matching and the mechanical strength that maintains a uniform air
(second) layer in the hi-lo-lo combinations. Waterhouse [7] suggested an electrically thin
superstrate (third layer) has the minimal effect on the overall antenna performance if the second
layer is made by foam. We noted, in previous publications [9]-[10], that the high-dielectric-constant
substrate of the CP-EMCP elements produces a tight radiation field and the coupling
between the substrate and the superstrate thus minimising the lateral (near-field) coupling
between elements, especially in the Parallel-plane coupling [9]. Note that we used Collinear- and
Parallel-plane for the CP antennas, instead of the traditional E- and H-plane for the LP
antennas. However, the effects of the superstrate thickness on the mutual coupling of the CP-EMCP
elements have not been evaluated yet. Hence in this paper, we focus our investigation on
the superstrate thicknesses to the performance of the CP-EMCP element.
